1. Field of the Invention
The present invention relates to a quartz crystal oscillator that includes a quartz crystal unit and an IC (integrated circuit) chip integrating an oscillator circuit using the crystal unit in one component, and more specifically, to a crystal oscillator with improved stability of oscillation frequency.
2. Description of the Related Arts
The crystal oscillator configured by combining the crystal unit and the IC chip integrating the oscillator circuit using the crystal unit into a single package component can easily adopt a surface mount configuration, and is widely incorporated as a reference source of frequency and time in a mobile electronic device, for instance, typified by a mobile phone.
In the crystal oscillator, the electronic circuit integrated in the IC chip is not limited to an oscillator circuit. Instead, for instance, an electronic circuit, such as a temperature compensating circuit, can be integrated into the IC chip. The temperature compensating circuit compensates temperature-frequency characteristics of the crystal unit and allows a constant oscillation frequency to be acquired irrespective of the ambient temperature. A crystal oscillator capable of highly accurately maintaining the oscillation frequency by integrating a temperature compensating circuit in the IC chip is referred to as a temperature compensated crystal oscillator (TCXO).
FIG. 1 shows an example of a circuit configuration of a TCXO.
The TCXO shown in the diagram includes: crystal unit 10; and IC chip 20 including an oscillator circuit using crystal unit 10. For instance, a crystal unit employing an AT-cut quartz crystal blank can be adopted as crystal unit 10. IC chip 20 includes: constant voltage circuit 21 that is supplied with a power supply voltage from the outside and in turn supplies a regulated internal power supply voltage to each circuit in IC chip 20; oscillator circuit 22 connected with crystal unit 10; frequency controller 23 that is supplied with an automatic frequency control (AFC) signal from the outside, generates a frequency control signal and supplies this signal to oscillator circuit 22; temperature sensor 24 that measures temperature; temperature compensating circuit 25 that generates a temperature compensating signal for compensating temperature-frequency characteristics of crystal unit 10 according to the measurement result of temperature by temperature sensor 24, and supplies this signal to oscillator circuit 22; nonvolatile memory 26 that stores data used for generating the temperature compensating signal; and output buffers 27 and 28 that amplify an oscillation output from oscillator circuit 22 and supply the output as an analog signal to external circuits. Output buffers 27 and 28 are arranged in parallel. Output buffer 28 thereof is capable of turning on and off according to an enable signal supplied from the outside. In consideration of a case of incorporating the TCXO into an electronic device having a wireless communication function, a continuous oscillation output signal from output buffer 27 is adopted as a reference signal for a PLL (phase-locked loop) synthesizer circuit in a wireless section, and further adopted as clocks for a DSP (digital signal processor) and a CPU (central processing unit) for system control. On the other hand, output buffer 28 is linked to an intermittent operation signal for the PLL circuit and allows on/off control of the output. An oscillation output signal from output buffer 28 is supplied to, for instance, circuits without need to operate during standby. By thus allowing the on/off control of the output from output buffer 28, power consumption of the electronic device with the crystal oscillator during standby is facilitated to be reduced.
Along with the above arrangement of circuits in IC chip 20, IC chip 20 includes: power supply terminal VDD to which power supply voltage is supplied; ground terminal GND; pair of crystal connecting terminals X1 and X2 electrically connected with crystal unit 10; input terminal AFC to which an automatic frequency control signal is supplied; enable terminal EN to which an enable signal is supplied; and two output terminals OUT1 and OUT2 at which oscillation output signals from respective output buffers 27 and 28 appear. A Colpitts oscillator circuit including an inverting amplifier is widely used as oscillator circuit 22. Crystal connecting terminal X1 is electrically drawn from an input side of the amplifier, and crystal connecting terminal X2 is electrically drawn from an output side of the amplifier. Accordingly, along with an operation of oscillator circuit 22, the phases of signal components of oscillation frequencies appearing at respective crystal connecting terminals X1 and X2 are opposite to each other as shown in FIG. 1. Both inverting and non-inverting amplifiers are usable as output buffers 27 and 28. However, as shown in FIG. 1, in a case where non-inverting amplifiers are employed as output buffers 27 and 28, signal phases at output terminals OUT1 and OUT2 are identical to the signal phase at crystal connecting terminal X2.
In a case of providing two output buffers in the crystal oscillator, one may be configured by an inverting amplifier and the other may be configured by a non-inverting amplifier. JP2005-026828A describes a crystal oscillator that employs inverting and non-inverting amplifiers as output buffers and allows simultaneous on/off control of outputs from the two output buffers. A configuration for acquiring two oscillation output signals that have the identical frequency but have phases opposite to each other is described in, for instance, US 2006/0176121 A1. In the crystal oscillator described in US 2006/0176121 A1, first and second amplifiers, both of which are inverting amplifiers, are employed, a crystal unit is inserted between an input terminal of the first amplifier and an input terminal of the second amplifier, a load capacitor is inserted between the input or output terminal of each amplifier and a ground point, the output terminal of the first amplifier and the input terminal of the second amplifier are connected to each other via a first dc blocking capacitor, the input terminal of the first amplifier and the output terminal of the second amplifier are connected to each other via a second dc blocking capacitor, and a feedback resistor is provided for each amplifier, allowing oscillation output signals having phases opposite to each other from the respective first and second amplifiers to be acquired.
Incidentally, what is configured by hermetically encapsulating a quartz crystal blank in a container is a crystal unit. Accordingly, the crystal oscillator is configured as a package component by integrating a container and an IC chip; this integration is made by means of fixing the IC chip to the container of the crystal unit, hermetically encapsulating the crystal blank and the IC chip in the same container, or using a container including two housing portions to hermetically house the crystal blank at one portion and house the IC chip at the other portion for integration. Also in this case, electronic circuits such as an oscillator circuit using a crystal blank are integrated in the IC chip. The crystal blank is represented as crystal unit 10 in a circuit diagram representation as shown in FIG. 1.